This invention relates to the production of ethers by reacting at least one alcohol with at least one mono-olefin having a double bond on a tertiary carbon atom.
This reaction is known to take place in the presence of acid catalysts, particularly solid ion exchange resins in the acid form, the best results being obtained by using macroreticular solid sulfonic resins, for example those disclosed in U.S. Pat. No. 3,037,052.
The alcohol is, for example, methanol or ethanol and the mono-olefin has a double bond on a tertiary carbon atom as, for example, isobutene, 2-methyl-1-butene, 2-methyl-2-butene, 2-methyl-1-pentene or 2-methyl-2-pentene. Mixtures of olefins may be used. The olefins of the above type, for example isobutene, are more reactive than the bi-secondary olefins, for example 2-butene, or the primary-secondary olefins, for example 1-butene, so that it is possible to treat mixtures of olefins. The olefins having a tertiary carbon atom react nearly exclusively, which is a way to eliminate these olefins from a hydrocarbon stream, for example, a C.sub.4 steam-cracking or catalytic cracking cut which may contain butadiene and/or saturated hydrocarbons.
The reaction of adding alcohols to olefins, which yields ethers, is a balanced and exothermic reaction.
It is thus necessary to eliminate the reaction heat, since the sulfonic resins do not withstand temperatures higher than 120.degree. C. over extended periods, and sudden heat thrusts are detrimental to the physical behaviour of the resin. It is also clear that high conversion rates are more easily attained when operating at a low temperature; however, the activity of the resin becomes a limiting factor.
Various techniques have been proposed to optimize this reaction of adding alcohols to olefins. It is thus known, for example, to pass the reactants in the liquid state through the particles of catalyst in fixed bed. It is found that, for reasons of physical behaviour of the resin and to avoid too high irreversible pressure drops due to the piling of the resin, it is desirable to arrange the catalyst as a number of low height catalyst layers and to cool the liquid when passing from a given layer to the next one. Another way to operate with a fixed bed is to pass the liquid through a number of externally cooled parallel tubes containing the catalyst. However, in that case, the reactor is complex and expensive, and it is difficult to avoid unequal distribution of the liquid flow through the tubes, resulting in unsatisfactory running of the reactor and accelerated decay of the resin.
The use of a reactor containing a catalyst dispersed throughout the liquid phase of the reactants does not provide for high olefin conversion rates, unless reactors of excessive volume are used.
It has also been proposed to operate in two serially arranged reactors with intermediary separation of the product or to use molar alcohol/olefin ratios higher than 1, in order to obtain increased conversion rates. In all these cases, the power necessary to distill either the hydrocarbon cut, for example a C.sub.4 cut, or methanol or other alcohol in excess to be recycled, is considerably increased.
It has also been proposed to operate with two successive catalyst beds (Patent application of the German Federal Republic No. 1,934,422). In the first bed, the catalyst is maintained dispersed in the liquid by the vaporization of one or more constituents of the liquid to dissipate a part of the reaction heat. The second bed consists of the catalyst accumulated in the bottom of the reactor. The temperature conditions are thus the same for the first bed and the second bed of the catalyst. The liquid is circulated downwardly.
It has been found that this technique has a great disadvantage: the vapor phase appears within the resin particle or at the contact surface thereof and forms an envelop therearound, thus impeding an easy access of the reactants, which leads to relatively poor conversions and selectivities and a reduced life of the catalyst. Another disadvantage is that the compound of lowest boiling point, the olefin in most cases, vaporizes and escapes from the reaction or, at the least, undesirably modifies the proportions of the reactants.